The present invention relates to a switching power supply (SPS), and more particularly to a packing structure and a packing method of a mini-size power supply.
Presently, for the development of electronic device, the volume trends to be smaller and smaller, and the current and power requirements trend to be bigger and bigger, especially for a high density switching power supply (SPS). Therefore, it is an important issue to achieve excellent heat-dissipating effect and reduce the current loading in such tiny space.
FIG. 1A is a diagram illustrating a switch circuit used in the switching power supply according to the prior art. Referring to FIG. 1A, an induced current is generated and inputted to a secondary winding 101 via a transformer 10. A set of rectifiers 102 and 103 then rectify the induced current to generate the output DC current to output through inductances 104 and 105 connected with a positive terminal (Vo) and a negative terminal (xe2x88x92Vo). The rectifiers 101 and 102 can either be a diode or a metal-oxide-semiconductor field effect transistor (MOSFET). FIG. 1B is a diagram illustrating an alternate circuit used in the switching power supply arranged slightly differently than that of FIG. 1A.
FIG. 2 is an exploded three-dimensional view illustrating a packed circuit in FIG. 1A which is a general packed structure used in the industry. Cores 200, 201 and a windings 202 shown in FIG. 2 is corresponded to the secondary winding 101 in FIG. 1A. Cores 203, 204 and a windings 207 shown in FIG. 2 are corresponded to the inductance 104 in FIG. 1A, and cores 205, 206 and a winding 208 shown in FIG. 2 are corresponded to the inductance 105 in FIG. 1A. The MOSFETs 211 and 212 are corresponded to the rectifiers 103 and 102 in FIG. 1A, respectively.
Currently, most rectifiers applied in SPS are MOSFETs. FIG. 3 is a perspective view illustrating the typical structure of a packed MOSFET. As shown in FIG. 3, a chip 301 is soldered onto a copper plate 300 which is a drain of the MOSFET. A source and a gate are bonding to two pins 302 and 304 via metal lines 306 and 305. After testing the electricity, the top of the chip 301 is packed by epoxy. Generally, the conductivity of copper is about 380 W/mk, while that of epoxy is smaller than 1 W/mk. For the general heat-dissipating mechanism, the MOSFET is connected onto a pad or a metal of a substrate by soldering or screwing, and a thermal pad is placed between the MOSFET and the substrate for heat-dissipating. Usually, the substrate is a FR4 printed circuit board and the metal is aluminum. Thus, the heat conduction pathway is to transfer the heat generated from the MOSFET to the pad of the substrate via the copper plate 300, and then dissipate the heat to the air by natural convection or forced convection.
On the other hand, most electric devices are soldered on the surface of substrate by the surface mounting technology (SMT). For SPS, the surface mounted device (SMD) is generally used in SPS designation. FIG. 4A is an exploded diagram illustrating a standard packed MOSFET bound to a printed circuit board according to the prior art. Generally, MOSFET has three pins, i.e. a gate 401, a source 402, and a drain 400 which is a copper plate. The copper plate 400 is soldered on a pad 404 of a printed circuit board 403, and the gate 401 and the source 402 are soldered on plates 405, 406 of the printed circuit board 403 respectively. After assembling, the structure is as shown in FIG. 4B. As shown in FIG. 4C, the heat conduction pathway is from the drain 400 located at the back of MOSFET to the printed circuit board 403 via a soldering material 407, i.e. the conductive materials such as tin or silver. Generally, since the material of printed circuit board is FR4 having conductivity of about 0.8 W/mk, the conduction effect is very small, i.e. the heat resistance is very large. Hence, most heat is directly transferred to the position just under the printed circuit board 403, i.e. under the MOSFET, by conduction, and dissipated into the air by convection as shown in FIG. 4C. Thus, if an electronic device which is not tolerance to heat such as capacitance is placed under the MOSFET, then the device lifetime will reduce because of high temperature generated by the MOSFET. However, since the device which could generate heat is soldered on the printed circuit board which is a FR4 material and is a bad conductor for heat, the generated heat is not easily taken away. According to the law of the conservation of energy, the temperature of the device which could generate heat will keep increasing because the generated heat cannot be dissipated, and further results in losing efficacy of the device because of the thermal run away effect.
In addition, for the power supply design having high current and high power characteristics, many MOSFETs are generally parallel connection for enhancing the efficiency, so the printed circuit board requires more thick copper line for loading larger current. Therefore, the space on the printed circuit board is occupied.
Summarily, the problems of the heat-dissipating effect, loading larger current and the space-consumption are still required to be solved in current industry. Therefore, the purpose of the present invention is to develop a method to deal with the above situations encountered in the prior art.
It is therefore an object of the present invention to propose a packing structure of a switching power supply for enhancing heat-dissipating effect.
It is therefore another object of the present invention to propose a packing structure of a switching power supply for loading and outputting more current.
It is therefore an additional object of the present invention to propose a packing structure of a switching power supply having the smaller packaged volume.
According to one aspect of the present invention, there is provided a packing structure of a switching power supply for enhancing heat-dissipating effect. The packing structure includes a printed circuit board, a transformer, an inductor having an inductive winding, a converter placed on a pad of the printed circuit board, wherein the pad is electrically connected to a secondary winding of the transformer and the inductive winding, and a metal cover directly covered on the converter.
Certainly, the metal cover can be made of copper.
Certainly, the converter can be a metal-oxide-semiconductor field effect transistor (MOSFET) having a drain directly connected to the metal cover and a source and a gate directly connected to the pad of the printed circuit board.
Preferably, the packing structure further includes a heatsink placed on the metal cover for enhancing heat-dissipating, or/and a thermal pad placed between the metal cover and the heatsink for conducting heat.
Preferably, the packing structure further includes a metal strip electrically connected to the metal cover, the inductive winding and the secondary winding of the transformer. The metal strip and the inductive winding can be integrally formed. The metal strip can be made of copper.
Certainly, the converter can be a diode having an anode electrically connected to the inductor and a cathode directly connected to a pad of the printed circuit board.
Certainly, the printed circuit board can be made of a material selected from FR4 and thermal clad.
According to another aspect of the present invention, there is provides a packing structure of a switching power supply for enhancing heat-dissipating effect. The packing structure includes a printed circuit board, a transformer, an inductor having an inductive winding, a metal strip electrically connected to the inductive winding, and a converter electrically connected to the metal strip and covered by the metal strip.
According to an additional aspect of the present invention, there is provides a method for packing a switching power supply to enhance heat-dissipating effect. The method includes steps of placing a converter on a pad of a printed circuit board, placing an inductor and a transformer on the printed circuit board, and electrically connecting a metal cover to the pad, an inductive winding of the inductor and a secondary winding of the transformer for enhancing heat-dissipating effect.